1. Field of the Invention
This invention relates to processes for the preparation of coated polyolefin films and more particularly to a process for preparing coated polyolefin films possessing excellent properties of heat-sealability, water vapor and oxygen gas impermeability, adhesive cellophane tape peeling strength of the coating and transparency.
2. Description of the Prior Art
One of the packaging films most widely used today is a laminated film made by laminating polyehtylene on cellophane. This combines the heat-sealability and water vapor barrier property of the polyethylene with the oxygen gas barrier property of the cellophane. However, when polyethylene is laminated on cellophane, not only does the price become higher, but also the excellent transparency of the cellophane is lost, the oxygen gas barrier property may still be insufficient and the range of use of the film product is limited. On the other hand, the heat-sealability depends on the thickness of the laminated polyethylene, but it far exceeds the usual practical strength requirements.
On the other hand, a biaxially stretched film of an isotactic polypropylene is noted for its excellent strength and transparency, but it is low in the oxygen gas barrier property and, because it is biaxially stretched and possesses a high crystallizability, thermal shrinkage thereof occurs at high temperatures and its heat-sealability is quite insufficient. Therefore, the uses of a biaxially stretched polypropylene film alone as a packaging film are considerably limited as compared with those of films of polyethylene laminated to cellophane.
The present inventors have suggested a process (which shall be referred to as a two-step coating process hereinafter) wherein a polypropylene film is coated with an undercoating consisting mostly of a chlorinated polypropylene resin (which shall be referred to as a graft polymer hereinafter) on which vinyl chloride or vinylidene chloride is graft-polymerized (the content of vinyl chloride or vinylidene chloride being 2 to 30% by weight based on the graft polymer) and with a top coating consisting mostly of a vinylidene chloride resin emulsion, and a process (which shall be referred to as a one-step coating process hereinafter) wherein the polypropylene film is coated with a composition consisting mostly of said graft polymer and vinylidene chloride resin (Japanese patent publication No. 9099/1974), to improve the heat-sealability and oxygen gas barrier property.
Furthermore, the present inventors have suggested (1) a one-step coating process (Japanese patent publication No. 8400/1972) wherein a composition consisting mostly of a mixture of a chlorinated polypropylene resin (which shall be referred to as a graft polymer hereinafter) on which a styrenic monomer is graft-polymerized and a vinylidene chloride resin, and (2) a two-step coating process (Japanese patent publication No. 36874/1972) wherein an under-coating consisting mostly of said graft polymer and a top-coating consisting mostly of a vinylidene chloride resin, are respectively applied to polypropylene films to improve the heat-sealability and oxygen gas barrier property.
However, in the above mentioned two-step coating process, when the surface coated with the under-coating consisting mostly of the graft polymer is coated with the vinylidene chloride resin emulsion as a top-coating, the adhesive cellophane tape peeling strength of the coating layer will be favorable, but the heat-sealing strength will be so insufficient that the uses of the thus-coated biaxially stretched polypropylene film alone will be considerably limited.